It is critical that military personnel timely identify enemy objects and combatants, and even more so as modern warfare occurs more frequently in urban areas. The urban environment presents situations where it becomes increasingly necessary to identify objects hidden behind walls and underfoot. In recent years, ground penetrating radar (GPR) has been crucial in the identification of subterranean objects such as improvised explosive devices (IEDs).
In order for the aforementioned radar to operate properly, modeling of the radar environment needs to accurately represent the materials encountered in-theater. Electromagnetic properties of materials in the environment determine the depth at which objects can be detected by the radar. One such property is permittivity (∈), which indicates the transmission, reflection, and absorption of EM radiation by a dielectric material. Since permittivity is an intrinsic material property that depends on temperature and water content, the ∈ for sand, soil, and vegetation varies by location and time-of-day. Thus, to accurately model the behavior of radar pulses in a test environment, permittivity must be measured as near to the radar test area as possible.
Several techniques exist for measuring the permittivity of dielectric samples. At radio frequencies (RF), two popular techniques are reflectometry and scatterometry. Reflectometry disadvantageously requires disturbing the sample by inserting probes. Also, a reflectometer, such as a pulse generator and oscilloscope, must be brought to the radar experiment site. Scatterometry disadvantageously requires that the measurement must be taken in the far-field, at a distance of at least several wavelengths from the sample. At this far-field distance, the sample must be several wavelengths in length and width for the measurement to be valid. An alternative method is to use ring resonators, which requires that a scalar network analyzer (or equivalent circuit) and a microstrip ring are brought to the experiment site. The ring-resonator measurement can be taken without disturbing the dielectric sample.
FIG. 1 is an illustration of a traditional circular ring resonator. The traditional ring resonator 125 is used in the measurement of permittivity of a dielectric sample 130 at frequencies above 1 GHz. The ring resonator 125 comprises an input feed line 100, a resonating ring 120, and an output feed line 115 disposed on a substrate (not shown). For measurement, a dielectric sample 130 is placed against the ring resonator 125. At the resonant frequency of the ring and multiples thereof, RF energy couples from the input feed line 100 to the ring 120 over a first gap 105, and from the ring 120 over a second gap 110 to the output via a second feed line 115. Permittivity is measured by comparing the resonance of the ring without a dielectric sample present to the resonance of the ring when a dielectric sample 130 is applied.
FIG. 2 is an illustration of applying a sample to a traditional circular ring resonator. The cutaway illustration of applies a sample material 205 flush onto the ring resonator 230 that is disposed on a dielectric substrate 235 and a ground plane 200. At resonance, a signal passes from the input feed line 210, couples over a first gap 220 to the ring 230, travels around the ring, and couples over a second gap 225 to the output feed line 215.
Given that the test sample 205 must be flush against the ring surface 230, it is increasingly difficult to ensure a flush fit at relatively low (e.g. below GHz) frequencies as the corresponding area of the ring and sample size increases. Thus, a traditional circular ring resonator is impractical for measuring permittivity below 1 GHz as the ring becomes prohibitively large (e.g. several feet across). These lower frequencies are used by GPR, though, because they offer deeper ground penetration using relatively low transmit power. Since accurate radar modeling must account for frequencies under 1 GHz, the permittivity of materials in the radar environment must be measured at such frequencies; thus, improved techniques are needed to measure permittivity.
A portable system may be useful to measure permittivity quickly in the field, with minimal disturbance of the dielectric sample, at ground penetrating frequencies.